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You are here: Home / Battery Chargers / Battery Cut Off Charger Circuit Using a Single Relay

Battery Cut Off Charger Circuit Using a Single Relay

Last Updated on June 15, 2019 by Swagatam 23 Comments

caution electricity can be dangerous

Amazed to hear this! Yep that’s actually possible, you would need only one relay and a handful of diodes to make a simplest one relay automatic battery charger circuit.

How it Works

The idea struck me while trying to design the easiest possible battery charger circuit for one my clients.

The concept is simple; just raise the operating or triggering voltage of the relay up to the optimal battery charging threshold voltage by dropping the required amount of supply voltage to the relay coil, with the help of series diodes.The idea may be understood from the following points:

Take an ordinary relay, measure its triggering voltage by carefully applying a variable voltage across its coil.Now suppose the triggering voltage of the particular relay was about 9 volts, and also assume you want to raise its voltage to 14 volts, which may be your 12 volt battery’s charging threshold voltage.

We know that a 1N4007 diode drops about 0.6 volts across it, so if we add sufficient number of diodes in series with the relay coil would hopefully pull its tripping or triggering voltage to about 14 volts.

That means, 14 – 9 = 5, we’ll require 5/0.5 = 10 diodes in series to achieve this rise in the triggering voltage of the relay.That’s pretty simple and interesting isn’t it?The rest may be done with the help of the shown diagram…..your simplest single relay automatic battery charger is ready.

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About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

Reader Interactions

Comments

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  1. Ajward says

    November 18, 2022

    Dear sir,
    Can we use resistor or a pot instead of diodes.

    Reply
    • Swagatam says

      November 18, 2022

      No, pot or resistive divider will not work instead of diodes.

      Reply
      • M.I.Ajward says

        November 20, 2022

        Dear sir,
        I would like to know whether it is Ok if I charge the 12v 36 ah car battery continuously with a 14v 1amp charger without auto cutoff circuit. I appreciate your reply.

        Reply
        • Swagatam says

          November 20, 2022

          Hello MI,
          yes it is perfectly OK. but with 1 amp your 36 Ah battery cannot charge properly, or might taken a very long time to get fully charged.

          Reply
  2. shrikant ranade says

    April 30, 2021

    I want some simple 5 volt relay projects with led
    i also purchase pcb boards of this project

    Reply
    • Swagatam says

      May 1, 2021

      You can use all the 12V relay projects from this website and convert them to 5V relay projects by replacing the 12V relay with a 5 V relay and replacing the 12V supply with 5 V supply

      Reply
  3. kavin says

    November 19, 2020

    How to design a relay to fix between laptop charger plug and brick and to control the relay using mobile phone

    Reply
  4. marwerno says

    June 30, 2020

    Hi,
    exactly what I was looking for (I was always thinking of a way with Z-Diodes).
    But I believe there is a small error?
    You write:
    “14 – 9 = 5, we’ll require 5/0.5 = 10 diodes”
    0.5 represents the Voltage drop of the diode, but in the N4001 example it would be 0.6 right?
    That would change the calculation to 8 Diodes?
    Thanks,
    Markus

    Reply
    • Swagatam says

      July 1, 2020

      Thank you for your feedback and suggestions!

      Reply
  5. Grtech says

    June 16, 2019

    Please share a automatic charging cut off circuit for 12v 35Ah battery.
    can I use 12v 2A tranfo.?

    Reply
    • Swagatam says

      June 16, 2019

      you can these circuits:
      https://www.homemade-circuits.com/high-current-10-to-20-amp-automatic/
      https://www.homemade-circuits.com/opamp-low-high-battery-charger/

      Reply
  6. untung suharto says

    February 10, 2017

    Hi, friend this is can workout well charge SLA battery?

    Reply
    • Swagatam says

      February 11, 2017

      yes it will work….

      Reply
  7. Farhan Ali says

    December 31, 2015

    yes relay starts to switch on and off. i have made this circuit.this is due to disconnection. it order to overcome this connect 14v dc to battery positive terminal not through the NC of relay.

    Reply
  8. Unknown says

    November 26, 2015

    Sir, thank you for sharing your idea on this simple circuit.
    What components will be required if I need this same simple circuit to charge 48 volts battery made from 4 x 12. The battery AH is not important.

    anticipating your quick response

    Thank you once more

    Reply
    • Swagatam says

      November 26, 2015

      sorry this circuit is not recommended for a 48V battery because too many diodes will be required for the 48v operation …….moreover the design is not too accurate.

      Reply
  9. Swagatam says

    February 23, 2014

    Actually, I was wrong in acknowledging the zener idea, it won't work.

    It has to be done exactly as suggested in the diagram in the above article. 1N4007 or 1N4148 in series with the relay coil is the only correct way of doing it.

    Reply
  10. Ahmed Elmasry says

    February 22, 2014

    hi i take your idea and make this circuit …

    im75.gulfup.com/3SLo5.png

    Reply
    • Swagatam says

      February 23, 2014

      Actually, I was wrong in acknowledging the zener idea, it won't work

      It has to be done exactly as suggested in the diagram in the above article. 1N4007 or 1N4148 in series with the relay coil is the only correct way of doing it.

      Reply
  11. Ahmed Elmasry says

    February 22, 2014

    i take your idea and made this circuit :im75.gulfup.com/3SLo5.png

    Reply
  12. rohit choudhary says

    December 22, 2013

    Please share a automatic cut off circuit so that , I can charge a mobile battery , with mobile charger.

    Reply
    • Swagatam says

      December 23, 2013

      you can try the second circuit published in this article:

      https://www.homemade-circuits.com/2012/07/make-6v-4ah-automatic-battery-charger.html

      Reply
  13. Swagatam says

    November 12, 2013

    12v input will not charge a 12V battery

    Reply

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